In recent years, fiber to the home (FTTh) and fiber to the desktop (FTTd) gradually become one development direction in the construction of communications networks and the construction of future optical fiber networks. Nowadays, conventional low water peak optical fibers (meeting ITU-T G.652C/D) have found wide applications in optical fiber access network and generally have a bend radius of 30 mm. In the configuration process of FTTx optical fibers, cabling of optical fibers is often needed indoors and in narrow environments. Optical fibers are subject to high bending stresses at a small bend radius, and especially in practical applications, optical fibers are often wound in storage boxes that become increasingly small. Therefore, it becomes necessary to develop an optical fiber having high bending resistance performance to meet the requirements for FTTx network cabling and device miniaturization. In November 2009 and June 2010, ITU-T has amended the bend insensitive G.657 optical fiber standard twice and added a research report on lifetime performance of optical fibers having small bend radiuses (‘Characteristics of a bending loss insensitive single-mode optical fiber and cable for the access network’ and Amendment 1: Revised Appendix 1-Lifetime expectation in case of small radius bending of single-mode fiber). The two times of amendments have basically specified different application targets of the G.657A1/A2 optical fiber and the G.657.B3 optical fiber in different bend radius use environments. The G.657.A1 optical fiber that meets the minimum bend radius of 10 mm is applicable to long-haul networks. The G.657.A2 optical fiber meets applications on the condition of a minimum bend radius of 7.5 mm and is mainly applied in metro networks and FTTh. The G.657.B3 optical fiber meets the use condition of a minimum bend radius of 5 mm, is mainly applied in fiber to the desktop (FTTd) and all-optical networks and used in the manner of indoor optical fiber/optical cable, and focuses on the service life problem of optical fibers in a bending condition.
Based on the specifications of ITU-T and the specific use environments and conditions of the G.657.B3 optical fiber, the G.657.B3 optical fiber is basically used for short distance communication transmission and focuses more on macro-bending performance at a small bend radius, and the compatibility with the G.652.D standard is not mandatory. However, because the G.652.D has been widely used in the optical communication field for decades, the use habits of most of the customers, optical fiber cabling habits, and peer equipment are designed on the basis of the G.652 optical fiber design. Therefore, the development of a G.657.B3 optical fiber compatible with the G.652 optical fiber standard further facilitates promotion and use of optical communications.
After years of researches, scientists and researchers all over the world have found that the mode field diameter and cut-off wavelength of an optical fiber play a major role in macro-bending loss of the optical fiber. A MAC value can qualitatively measure the bending performance of an optical fiber, in which MAC is defined as a ratio between a mode field diameter and a cut-off wavelength. When the MAC is smaller, the bending performance of the optical fiber is higher. Apparently, the object of lowering the MAC can be achieved by lowering a mode field diameter and increasing a cut-off wavelength of an optical fiber, so as to obtain high bending performance. U.S. publications No. 2007/007016, and Chinese patent Nos. CN1971321A, and CN1942793A adopt this type of methods. However, when a mode field diameter of an optical fiber is too small, a large connection loss occurs in its connection with a conventional single-mode optical fiber, and the incident optical power is limited. Also, in consideration of a multi-service characteristic of FTTx, it is expected to use the full band for transmission, and the cut-off wavelength of the optical cable has to be smaller than 1260 nm. Therefore, the space for the cut-off wavelength of the optical fiber to increase is very limited. High bending performance cannot be effectively obtained only depending on the method of lowering a MAC value of an optical fiber to meet the G.657.B3 standard requirements.
In contrast to the ordinary sectional structure of the single-mode optical fiber, another effective method of enhancing the bending performance of an optical fiber is to adopt a design of a depressed inner cladding layer. For example, the design of a depressed inner cladding layer is adopted in U.S. Pat. Nos. 5,032,001, 7,043,125, and Chinese patent No. CN176680. Through the design of a depressed inner cladding layer, the numerical aperture (NA) of an optical fiber can be increased without increasing doping in the core layer, so as to avoid the increase of attenuation caused by increased doping. However, the optimized design of a depressed inner cladding layer can only improve the macro-bending performance of an optical fiber at a large bend radius to a certain extent. When the bend radius of an optical fiber is smaller than or equal to 10 mm, it is very difficult to adopt the method of a depressed inner cladding layer to prepare a bend insensitive optical fiber that meets the G.657.A2 standard.
It is found through further researches that the most effective method of enhancing bending resistance performance of an optical fiber is to design the cross-section of an optical fiber by adopting a structure of a trench cladding layer, the basic waveguide structure thereof is described in U.S. Pat. Nos. 4,852,968 and 6,535,679 and Chinese patent No. CN1982928A also adopt the same type of design. However, all the above patents only consider how to lower a bending induced loss and none considers a long service life of the optical fiber at a small bend radius in combination with specific applications, and also none explicitly illustrates whether an optical fiber fabricated according to the specification thereof meets or goes beyond the relevant requirement of a minimum bend radius of 5 mm in the G.657.B3 standard. It is found through the research on an optical fiber having the structure of a trench cladding layer that certain requirements and limitations also exist about the depth and width of a trench cladding layer in the cross-section of an optical fiber: if the trench cladding layer is too shallow or too narrow, the desirable bend insensitive performance is not achieved; and if too deep or too wide, the cut-off wavelength and dispersion performance of an optical fiber might be affected.
In a bend insensitive optical fiber having the structure of a trench cladding layer, another factor that affects the macro-bending performance of an optical fiber in a bending condition is a core/cladding layer diameter ratio of the optical fiber. When the optical fiber is in a bending state, as the circular cladding of the inner cladding layer envelops the core layer, the stress generated from bending first acts on the inner cladding layer and is then transferred to the core layer part. Without considering factors such as the core layer, cladding layer doping, and refractive index, a small core layer/cladding layer diameter ratio helps to enhance the bending performance of an optical fiber. However, a small core layer/cladding layer diameter ratio usually also affects performance such as MFD and dispersion of an optical fiber, and the matching of viscosity and stress also becomes more difficult in the drawing process. Therefore, a suitable core layer/cladding layer diameter ratio is also an important direction in researches on the cross-section of the G.657.B3 optical fiber. The latest researches indicate that: in an optical fiber link, especially an FTTx link, due to the existence of multiple bends and connectors, the phenomenon of a multi-path interference (MPI) might occur in the optical fiber. David. Zhen et al. has introduced the method of testing an MPI in OFC/NFOEC (‘Testing MPI Threshold in Bend Insensitive Fiber Using Coherent Peak-To-Peak Power Method’) in 2009. Especially in the optical fiber design of a trench cladding layer, if the depressed cladding layer is too close to the core layer, once a core layer offset occurs at an connector of an optical fiber, multi-path interferences occur easily. If the depressed cladding layer is too far away from the core layer, the effect of lowering the bending induced loss of the optical fiber cannot be achieved. Therefore, it is necessary to perform precise positioning on the depressed cladding layer. Hence, to properly design the cross-section of an optical fiber and obtain a desirable balance in the refractive index sectional structure of a core layer, a cladding layer, and a trench cladding layer is a focus and a challenge in the research of the G.657.B3 optical fiber.
In addition, during the use of an access network, apart from adopting a splicing method for an optical fiber connection, a mechanical connection manner may further be adopted. For example, an optical fiber mechanical connecting terminal requires that an optical fiber has a good end face quality after being cut; therefore, the optical fiber needs to have desirable material homogeneity.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.